In the design of low-power receivers there are several constraints. The main one is to reach the best possible performance while consuming the least power possible.
To insure a low power consumption, the design must be as simple as possible. Direct detection (i.e. without intermediate frequency processing, and conversion to baseband in a single step) is the simplest kind of demodulation chain and also the one which consumes the least power.
However, the detected signal is typically not strong enough to allow demodulation, so that it must be amplified.
The low-power circuitry imposes a second main constraint which is the leakage current.
By way of example, a typical desired targeted current consumption of a receiver can be in the range of a few μA, and the temperature range over which it must operate can range from −55° C. up to 125° C. A submicron CMOS process is for example used for this circuit.
In order to achieve a high detection gain, an active RF detector is used. The principle of operation of this type of detector relies on the non-linear transfer characteristic of active device. For example, an exponential relationship governs MOS transistors in the weak inversion regime.
Unlike detectors realised with isolated diodes, this type of active detector does not provide easily complementary signals. These signals are nevertheless very useful in order to insure high gain amplification (DC offset cancellation). There is therefore a need to generate a reference voltage with which the detected signal will be compared in order to generate a differential output, in particular using a differential amplifier. This presents a first problem in the circuit design.
A second problem relates to the DC offset of the differential amplifier that processes the detected signal and its reference. This DC offset depends on process parameters and mismatch between components. A wrong differential voltage applied to the inputs of the differential amplifier can result in the output being locked to a high or low value. In this case, the signal is lost.
The applied differential voltage must be adjusted taking into account process and mismatch variations to insure the proper operation of the system.
Usually the problems outlined above are solved independently. To solve the problem of incompatibility between a single output detector and a differential input amplifier, an envelope detector with two outputs can be used. One output provides the detected signal and the second one provides a voltage level which is used as a reference.
Unfortunately this kind of design requires duplication of the detector circuit, and therefore its associated current consumption.
The DC offset issue of the amplifier is generally minimized by using large components which in practice are not suitable for ultra-low-power circuitry. Indeed these components occupy a large silicon area. Apart from cost, this makes them subject to significant current leakage that could be of the same order of magnitude of their own current consumption. This prevents a proper operation.